Vertical cavity surface emitting lasers (VCSELs) have proven to be high performance optical sources for low cost, fiber optic communication systems. VCSELs can be made in one of four ways: etched post, implanted, oxide-confined, and planar-buried. These four families are distinguished by the nature of the electrical confinement that is used. At this time, only the implanted and oxide-confined VCSELs have been proven to be technologically and economically viable.
Implanted VCSELs suffer from a poorly defined current aperture and in most implementations, they lack an index guided optical aperture. The current guide is poorly defined due to the statistically diffuse nature of the implantation, which results in a gradual change from conductive to insulating material. Thus, high efficiency devices with apertures smaller than ˜5 microns have not been demonstrated. In contrast, efficient oxide-confined VCSELs have been demonstrated with apertures less than ˜2 microns. This improvement is possible because the native oxide used to confine the current can be precisely placed both laterally and vertically. In addition, the oxide layer also provides optical confinement of the lasing mode arising from the large index contrast between the oxide layer and the surrounding semiconductor.
All VCSELs operate in one or more optical modes. When an index guide exists, the lateral optical mode of the structure can be well described using standard electro-magnetic models. Thus for oxide-confined devices, the modal properties of the devices can be predicted. The modes are well-defined even at current densities just above threshold due to the large index contrast provided by the oxide aperture.
In comparison, the implanted VCSELs lack an inherent optical index guide. Once current flows through the implanted VCSEL, a thermally induced index profile forms, due to a temperature profile created by current flowing through the device, since the structure has a non-zero resistance. Before the formation of the thermal index guide, the device may lase in a poorly defined, unstable manner characterized by one or more lasing “spots”, localized regions substantially smaller than the implant (or oxide) aperture with intense light emission. This type of lasing is believed to be initiated by random, localized depressions in the carrier density, which result in small optical index steps. Because of the localized index step, lasing in this region is initiated, which stabilizes this fluctuation. This step contributes to the retention of the depletion of the carrier density in the lasing region. The formation of these lasing spots is a random process and is inherently unstable. Therefore, the spots may move or hop around spatially within the device aperture. In practice, these spots also manifest themselves as kinks in the light-output (L) versus current (I) characteristics. Kinks are discontinuities in the slope of the LI curve.
VCSELs operate in low divergence angle optical modes that can be easily coupled into optical fiber used in fiber optic data communications. In the ideal case, a high performance VCSEL device will couple consistently into the optical fiber and the coupling efficiency will not be a function of bias, and furthermore a high performance device will also have a modal structure that will lend itself to low noise operation.
At different bias currents, VCSELs possess different optical modal properties that are evident from the spectral output of the devices, as well as both the near and far field profiles. During high frequency modulation, the modal properties of the devices may change as a function of time as a result of a transient change in bias current, e.g. a switch from a high to low state. If the fiber coupling efficiency of the different modes is not identical, these modal changes will manifest themselves as variations with respect to time in the optical eye diagram. If the coupling is initially poor, and then improves with time, the device will appear to have a very slow rise time, which will lead to eye closure. Alternatively, if the coupling is initially good and then degrades with time, it will manifest itself as excessive overshoot in the eye diagram. During the turn-off of the eye equivalent problems can result. Typically, this coupling efficiency variation is reduced when a VCSEL operates in a large number of modes. Since none of the modes are dominant, the variations between modes are minimized. Unfortunately, in optical communication applications, lasers are modulated between two states: high (“1”) and low (“0”). The bias current corresponding to the low state, is just above threshold where VCSELs typically operate in lower order modes. This results in non-ideal optical properties. During transitions between the low and high state, the VCSEL may change between operating in a few low order modes to operating in many high order modes, and as a result coupling (efficiency) variations can result.
A useful VCSEL should also operate with low noise. Here noise refers to the appearance of random intensity fluctuations as measured by the receiver in an optical fiber data link. One important source of noise is due to the beating of a few modes on the surface of the photodiode after passing through the optical fiber. When many modes are present, this type of noise is minimized, but when only two or three modes are present this type of noise is more significant. Additionally, for this beat noise to be significant there needs to be some sort of variation to cause the beating. One source of variation arises from the spatial instability of the intensely lasing spots exhibited by implanted VCSELs operating at currents just above threshold.
Near threshold at the low state, implanted VCSELs typically operate in no more than three modes and may operate in unstable lasing spots. At the high bias level, the devices will operate in many higher order modes. These characteristics are not optimal and result in coupling efficiency variations and beat noise that degrade the optical eye diagrams.
Oxide-confined VCSELs experience similar behavior with the exception that they are less susceptible to unstable spot formation because of the well-defined optical index guide provided by the oxide aperture.
Because the optical properties of the VCSELs are effected by changes in bias, operating temperature, and the alignment between the gain and the Fabry-Perot, e.g. lasing mode, it is extremely difficult to force the device to exhibit the desired optical modal properties over the entire operating range including temperature and bias.
U.S. Pat. No. 5,774,487, issued on Jun. 30, 1998, entitled “Filamented Multi-Wavelength Vertical Cavity Surface Emitting Laser”, Morgan, et al. reduced the noise by creating an array of very small lasers within an aperture, e.g. lasing in filaments. Filamented light output is defined to be one that differs from multi-mode light output in that each element operates like a separate laser substantially independent from the other filaments. These filaments are fixed spatially and result in many incoherent independent lasing sources. When operating in filaments, each isolated section of the laser will lase independently of every other portion of the laser. Since the filaments are small, they will lase primarily in the fundamental mode. Thus, even though many filaments are lasing and there are many spectral lines, the far field pattern is essentially Gaussian. This structure has a highly stable modal pattern made up of many independent or incoherent single mode spatially fixed filaments. Thus, the coupling into the optical fiber is consistent with respect to bias and temperature, resulting in very small coupling variations and low beat noise.
In spite of the low noise operation, these devices are not optimum for fiber optic links due to their excessive linewidth arising from the lack of coupling between independent spatially fixed filaments. If the linewidth is too large, then the bandwidth distance product of the link is compromised due to chromatic dispersion. Standards, e.g. Gigabit Ethernet and Fibre Channel, both define linewidth and specify maximum allowable linewidths to ensure that the signal can be read over the required distances of fiber. The linewidths of the VCSELs described by Morgan, et al. are too broad for current and proposed high performance standards needed at frequencies greater than 1 Gb/s. Even narrower linewidths may be required for higher speed or longer links applications making the techniques taught by Morgan, et al. inappropriate.